Effective Java: Concurrency

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Effective JavaConcurrency

• Last Updated Fall 2009

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Agenda

• Material From Joshua Bloch
• Effective Java Programming Language Guide
• Cover Items 66-73
• Concurrency Chapter
• Bottom Line
• Primitive Java concurrency is complex

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Item 66 Synchronize Access to Shared Mutable Data

• Method synchronization yields atomic transitions
• public synchronized boolean doStuff()
• Fairly well understood
• Method synchronization also ensures that other
  threads see earlier threads
• Not synchronizing on shared atomic data
  produces wildly counterintuitive results
• Not well understood

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Item 66 Unsafe Example

• // Broken! How long do you expect this program
  to run?
• public class StopThread
• private static boolean stopRequested
• public static void main (String args)
• throws InterruptedException
• Thread backgroundThread new Thread(new
  Runnable()
• public void run() // May
  run forever!
• int io while (! stopRequested) i
  // See below
• )
• backgroundThread.start()
• TimeUnit.SECONDS.sleep(1)
• stopRequested true
• // Hoisting transform
• // while (!loopTest) i ? if (!loopTest)
  while(true) i
• // Also note anonymous class

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Item 66 Fixing the Example

• // As before, but with synchronized calls
• public class StopThread
• private static boolean stopReq
• public static synchronized void setStop()
  stopReq true
• public static synchronized void getStop()
  return stopReq
• public static void main (String args)
• throws InterruptedException
• Thread backgroundThread new Thread(new
  Runnable()
• public void run() // Now sees main
  thread
• int io while (! getStop() ) i
• )
• backgroundThread.start()
• TimeUnit.SECONDS.sleep(1)
• setStop()
• // Note that both setStop() and getStop() are
  synchronized

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Item 66 A volatile Fix for the Example

• // A fix with volatile
• public class StopThread
• // Pretty subtle stuff, using the volatile
  keyword
• private static volatile boolean stopRequested
• public static void main (String args)
• throws InterruptedException
• Thread backgroundThread new Thread(new
  Runnable()
• public void run()
• int io while (! stopRequested) i
• )
• backgroundThread.start()
• TimeUnit.SECONDS.sleep(1)
• stopRequested true

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Item 66 volatile Does Not Guarantee Mutual
Exclusion

• // Broken! Requires Synchronization!
• private static volatile int nextSerialNumber 0
• public static int generateSerialNumber()
• return nextSerialNumber
• Problem is that the operator is not atomic
• // Even better! (See Item 47)
• private static final AtomicLong nextSerial new
  AtomicLong()
• public static long generateSerialNumber()
• return nextSerial.getAndIncrement()

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Item 66 Advice on Sharing Data Between Threads

• Confine mutable data to a single Thread
• May modify, then share (no further changes)
• Called Effectively Immutable
• Allows for Safe Publication
• Mechanisms for safe publication
• In static field at class initialization
• volatile field
• final field
• field accessed with locking (ie synchronization)
• Store in concurrent collection (Item 69)

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Item 67 Avoid Excessive Synchronization

• // Broken! Invokes alien method from sychronized
  block
• public interface SetObltEgt void
  added(ObservableSetltEgt set, E el)
• public class ObservableSetltEgt extends
  ForwardingSetltEgt // Bloch 16
• public ObservableSet(SetltEgt set) super(set)
  
• private final ListltSetObltEgtgt obs new
  ArrayListltSetObltEgtgt()
• public void addObserver (SetObsltEgt ob )
• synchronized (obs) obs.add(ob)
• public boolean removeObserver (SetObltEgt ob )
• synchronized (obs) return obs.remove(ob)
  
• private void notifyElementAdded (E el)
• synchronized(obs) for (SetObltEgt obobs) //
  Exceptions?
• ob.added(this, el)
• _at_Override public boolean add(E el) // from
  Set interface
• boolean added super.add(el)
• if (added) notifyElementAdded (el)
• return added

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More Item 67 Whats the Problem?

• public static void main (String args)
• ObservableSetltIntegergt set new
  ObservableSetltIntegergt
• (new HashSetltIntegergt)
• set.addObserver (new SetObltIntegergt()
• public void added (ObservableSetltIntegergt
  s, Integer e)
• System.out.println(e)
• if (e.equals(23)) s.removeObserver(this)
  // Oops! CME
• // See Bloch for a variant that
  deadlocks instead of CME
• )
• for (int i0 i lt 100 i)
• set.add(i)

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More Item 67 Turning the Alien Call into an Open
Call

• // Alien method moved outside of synchronized
  block open call
• private void notifyElementAdded(E el)
• ListltSetObltEgtgt snapshot null
• synchronized (observers)
• snapshot new ArrayListltSetObltEgtgt(obs)
• for (SetObserverltEgt observer snapshot)
• observer.added(this, el) // No more CME
• Open Calls increase concurrency and prevent
  failures
• Rule Do as little work inside synch block as
  possible
• When designing a new class
• Do NOT internally synchronize absent strong
  motivation
• Example StringBuffer vs. StringBuilder

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Item 67 Alternate Fix Using CopyOnWriteArray

• public interface SetObltEgt void
  added(ObservableSetltEgt set, E el)
• public class ObservableSetltEgt extends
  ForwardingSetltEgt // Bloch 16
• public ObservableSet(SetltEgt set) super(set)
  
• private final ListltSetObltEgtgt obs new
• CopyOnWriteArrayListltSetObltEgtgt()
• public void addObserver (SetObsltEgt ob )
• synchronized (obs) obs.add(ob)
• public boolean removeObserver (SetObltEgt ob )
• synchronized (obs) return obs.remove(ob)
  
• private void notifyElementAdded (E el)
• for (SetObltEgt obobs) // Iterate on copy
  No Synch!
• ob.added(this, el)
• _at_Override public boolean add(E el) // from
  Set interface
• boolean added super.add(el)
• if (added) notifyElementAdded (el)
• return added

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Item 68 Prefer Executors and Tasks to Threads

• Old key abstraction Thread
• Unit of work and
• Mechanism for execution
• New key abstractions
• Task (Unit of work)
• Runnable and Callable
• Mechanism for execution
• Executor Service
• Start tasks, wait on particular tasks, etc.
• See Bloch for references

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Item 69 Prefer Concurrency Utilities to wait and
notify

• wait() and notify() are complex
• Java concurrency facilities much better
• Legacy code still requires understanding low
  level primitives
• Three mechanisms
• Executor Framework (Item 68)
• Concurrent collections
• Internally synchronized versions of Collection
  classes
• Extensions for blocking, Example BlockingQueue
• Synchronizers
• Objects that allow Threads to wait for one another

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More Item 69 Timing Example

• // Simple framework for timing concurrent
  execution
• public static long time (Executor executor, int
  concurrency, final Runnable action) throws
  InterrruptedExecution
• final CountDownLatch ready new
  CountDownLatch(concurrency)
• final CountDownLatch start new
  CountDownLatch(1)
• final CountDownLatch done new
  CountDownLatch(concurrency)
• for (int i0 ilt concurrency i)
• executor.execute (new Runnable()
• public void run() ready.countDown()
  // Tell Timer were ready
• try start.await() action.run()
  // Wait till peers are ready
• catch (...) ...
• finally done.countDown() //
  Tell Timer were done
• )
• ready.await() // Wait for all workers
  to be ready
• long startNanos System.nanoTime()
• start.countDown() // And theyre off!
• done.await() // Wait for all workers
  to finish
• return System.nanoTime() startNanos

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Item 70 Document Thread Safety

• Levels of Thread safety
• Immutable
• Instances of class appear constant
• Example String
• Unconditionally thread-safe
• Instances of class are mutable, but is internally
  synchronized
• Example ConcurrentHashMap
• Conditionally thread-safe
• Some methods require external synchronization
• Example Collections.synchronized wrappers
• Not thread-safe
• Client responsible for synchronization
• Examples Collection classes
• Thread hostile Not to be emulated!

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Item 71 Use Lazy Initialization Judiciously

• Under most circumstances, normal initialization
  is preferred
• // Normal initialization of an instance field
• private final FieldType field
  computeFieldValue()
• // Lazy initialization of instance field
  synchronized accessor
• private FieldType field
• synchronized FieldType getField()
• if (field null)
• field computeFieldValue()
• return field

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More Item 71 Double Check Lazy Initialization

• // Double-check idiom for lazy initialization of
  instance fields
• private volatile FieldType field // volatile
  key see Item 66
• FieldType getField()
• FieldType result field
• if (result null) // check with no
  locking
• synchronized (this)
• result field
• if (result null) // Second check with
  a lock
• field result computeFieldValue()
• return result

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Item 72 Dont Depend on the Thread Scheduler

• Any program that relies on the thread scheduler
  is likely to be unportable
• Threads should not busy-wait
• Use concurrency facilities instead (Item 69)
• Dont Fix slow code with Thread.yield calls
• Restructure instead
• Avoid Thread priorities

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Item 73 Avoid Thread Groups

• Thread groups originally envisioned as a
  mechanism for isolating Applets for security
  purposes
• Unfortunately, doesnt really work